feat(client): list TAP adapters before connecting

Add a --list-tap-adapters mode to the Windows client. The command prints the
TAP-Windows6 adapter instance ids and exits without requiring relay, room, or
certificate arguments.

This makes the manual MVP test smoother on machines with multiple TAP adapters:
the operator can ask the binary for the exact ids, then rerun with
--tap-instance-id instead of relying on a separate PowerShell query or waiting
for the ambiguous-adapter startup error.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-client-win
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md Windows client TAP adapter responsibility

README.md

@@ -227,7 +227,8 @@ reinstalled before it reloads the configured `NetworkAddress`.
 If exactly one TAP-Windows6 adapter is installed, the client opens it
 automatically. If multiple TAP-Windows6 adapters are installed, startup fails
 until `--tap-instance-id` selects the intended adapter by NetCfgInstanceId /
-InterfaceGuid.
+InterfaceGuid. `--list-tap-adapters` prints the TAP adapter ids and exits
+without connecting.
 It prints and reports client diagnostics snapshots with relay reachability,
 LAN-gateway presence, route-pinning, QUIC datagram budget, relay RTT, TAP
 status/IP, broadcast frame flow, frame/datagram counters, and drops. The

TESTING.md

@@ -104,8 +104,7 @@ If the Windows machine has multiple TAP-Windows6 adapters, select the intended
 one explicitly:
 
 ```powershell
-Get-NetAdapter | Where-Object InterfaceDescription -Like "*TAP*" |
-  Select-Object Name, InterfaceGuid, InterfaceDescription
+.\target\release\lanparty-client-win.exe --list-tap-adapters
 
 .\target\release\lanparty-client-win.exe `
   --relay relay.example.net:8443 `

crates/lanparty-client-win/src/main.rs

@@ -20,7 +20,6 @@ use lanparty_client_route::{
 };
 #[cfg(windows)]
 use lanparty_client_tap::TapAdapter;
-#[cfg(any(windows, test))]
 use lanparty_client_tap::TapAdapterInfo;
 use lanparty_ctrl::{ControlMessage, PeerInfo, Role, RoomCode};
 use lanparty_net::RelayEndpoint;
@@ -42,21 +41,33 @@ const CLIENT_DIAGNOSTICS_INTERVAL: Duration = Duration::from_secs(10);
     about = "Windows TAP client for the LAN party L2 tunnel"
 )]
 struct ClientArgs {
+    /// List TAP-Windows6 adapter ids and exit without connecting.
+    #[arg(long)]
+    list_tap_adapters: bool,
+
     /// Relay DNS name or UDP socket address; bare hosts default to UDP/443.
-    #[arg(long, value_name = "HOST[:PORT]")]
-    relay: RelayEndpoint,
+    #[arg(
+        long,
+        value_name = "HOST[:PORT]",
+        required_unless_present = "list_tap_adapters"
+    )]
+    relay: Option<RelayEndpoint>,
 
     /// TLS server name expected in the relay certificate.
     #[arg(long, default_value = "lanparty-relay.local")]
     server_name: String,
 
     /// DER-encoded relay CA/certificate to trust.
-    #[arg(long, value_name = "PATH")]
-    relay_ca_cert: PathBuf,
+    #[arg(
+        long,
+        value_name = "PATH",
+        required_unless_present = "list_tap_adapters"
+    )]
+    relay_ca_cert: Option<PathBuf>,
 
     /// Room code to join as a remote client.
-    #[arg(long)]
-    room: RoomCode,
+    #[arg(long, required_unless_present = "list_tap_adapters")]
+    room: Option<RoomCode>,
 
     /// Identity JSON file used to persist the generated virtual MAC.
     #[arg(
@@ -87,10 +98,17 @@ struct ClientRuntimeConfig {
 
 impl ClientArgs {
     fn into_config(self) -> Result<ClientRuntimeConfig> {
-        let relay_ca_cert = fs::read(&self.relay_ca_cert).with_context(|| {
+        if self.list_tap_adapters {
+            bail!("--list-tap-adapters exits before building a tunnel config");
+        }
+
+        let relay = self.relay.context("--relay is required")?;
+        let relay_ca_cert_path = self.relay_ca_cert.context("--relay-ca-cert is required")?;
+        let room = self.room.context("--room is required")?;
+        let relay_ca_cert = fs::read(&relay_ca_cert_path).with_context(|| {
             format!(
                 "failed to read relay CA certificate {}",
-                self.relay_ca_cert.display()
+                relay_ca_cert_path.display()
             )
         })?;
 
@@ -99,16 +117,15 @@ impl ClientArgs {
             None => ClientIdentityStore::new(self.identity_file)?.load_or_create()?,
         };
 
-        let relay_addr = self
-            .relay
+        let relay_addr = relay
             .resolve()
-            .with_context(|| format!("failed to resolve relay endpoint {}", self.relay))?;
+            .context("failed to resolve relay endpoint")?;
 
         let session = ClientSessionConfig::new(
             relay_addr,
             self.server_name,
             relay_ca_cert,
-            self.room,
+            room,
             identity.virtual_mac(),
             self.max_datagram_size,
         )?;
@@ -122,7 +139,13 @@ impl ClientArgs {
 
 #[tokio::main]
 async fn main() -> Result<()> {
-    let config = ClientArgs::parse().into_config()?;
+    let args = ClientArgs::parse();
+    if args.list_tap_adapters {
+        print_available_tap_adapters()?;
+        return Ok(());
+    }
+
+    let config = args.into_config()?;
     println!(
         "lanparty-client-win connecting virtual MAC {} to relay {} room {}",
         config.session.virtual_mac(),
@@ -165,6 +188,15 @@ async fn main() -> Result<()> {
     run_result
 }
 
+fn print_available_tap_adapters() -> Result<()> {
+    let adapters = lanparty_client_tap::available_adapters()?;
+    for line in format_tap_adapter_list(&adapters) {
+        println!("{line}");
+    }
+
+    Ok(())
+}
+
 #[cfg(windows)]
 async fn run_client(
     session: &ClientSession,
@@ -488,6 +520,29 @@ fn available_tap_adapter_label(adapters: &[TapAdapterInfo]) -> String {
         .join(", ")
 }
 
+fn format_tap_adapter_list(adapters: &[TapAdapterInfo]) -> Vec<String> {
+    if adapters.is_empty() {
+        return vec!["No TAP-Windows6 adapters found".to_owned()];
+    }
+
+    let mut lines = Vec::with_capacity(adapters.len() + 1);
+    lines.push("TAP-Windows6 adapters:".to_owned());
+    lines.extend(adapters.iter().map(|adapter| {
+        let driver_key = adapter
+            .driver_key_name()
+            .map(|driver_key| format!(" driver-key={driver_key}"))
+            .unwrap_or_default();
+        format!(
+            "  {} component={}{}",
+            adapter.instance_id(),
+            adapter.component_id(),
+            driver_key
+        )
+    }));
+
+    lines
+}
+
 fn client_diagnostics_snapshot(
     session: &ClientSession,
     route_pinned: bool,
@@ -1059,8 +1114,19 @@ mod tests {
             "ROOM1",
         ]);
 
-        assert_eq!(args.relay.host(), "relay.example.test");
-        assert_eq!(args.relay.port(), DEFAULT_RELAY_PORT);
+        let relay = args.relay.unwrap();
+        assert_eq!(relay.host(), "relay.example.test");
+        assert_eq!(relay.port(), DEFAULT_RELAY_PORT);
+    }
+
+    #[test]
+    fn accepts_tap_adapter_listing_without_tunnel_args() {
+        let args = ClientArgs::parse_from(["lanparty-client-win", "--list-tap-adapters"]);
+
+        assert!(args.list_tap_adapters);
+        assert!(args.relay.is_none());
+        assert!(args.relay_ca_cert.is_none());
+        assert!(args.room.is_none());
     }
 
     #[test]
@@ -1129,6 +1195,21 @@ mod tests {
         assert!(error.contains("tap-two"));
     }
 
+    #[test]
+    fn formats_tap_adapter_listing() {
+        assert_eq!(
+            format_tap_adapter_list(&[]),
+            vec!["No TAP-Windows6 adapters found".to_owned()]
+        );
+        assert_eq!(
+            format_tap_adapter_list(&[tap_info("tap-one")]),
+            vec![
+                "TAP-Windows6 adapters:".to_owned(),
+                "  tap-one component=tap0901".to_owned(),
+            ]
+        );
+    }
+
     #[test]
     fn formats_relay_lifecycle_events() {
         let gateway = ControlMessage::PeerJoined(PeerInfo::new(1, Role::Gateway, None).unwrap());